That's not quite true. It really depends on what country, origins of the work, etc. i.e. this is the kind of nuance that warrants asking a copyright lawyer.
There's a few things here:
1) Is the work copyrighted?
2) Can you make money off of it?
3) Is it enforceable?
Here's from my limited knowledge of the US's copyright system.
1) Is the work copyrighted?
First off, a copyright is just as the name says. The "right" to "copy". Even if not officially copyrighted, by US standards, any unique work is the intellectual property of the creator. As such, copyright rules can apply. A good example would be if someone was to post an original story online. Even if a famous author were to come along and uses the character, names and plot, the original author has grounds to sue and demand compensation. (Provided the system works as the law describes, of course.) As far as I know, most countries that adhere that respect international copyrights will adhere to this standard to some capacity. Again, please refer to a copyright lawyer for final say as I am not one. So, the answer is, creative works doesn't have to be registered to enjoy some copyrights. But, of course, rules and restrictions may apply. IP law is pretty messy. -_- The one big exception to this is if the IP owner declares the IP to be creative commons and/or public domain. In which, it's fair game.
2) Can you make money off of it?
Yes and no. If you were to claim the works as your own, then no. US law treats it as a kind of fraud because you're selling/trading on someone else's property, possibly fooling the consumer into thinking that you and/or your products are the same as the original creator and their product. However, commissioning/asking someone to make a copy of something for yourself is a little different. Since the person that's asking for X to be made knows that the creator for X isn't the IP owner, there's no fraud involved. This is why there are patreons for people creating parody porn stuff. Though this has been both challenged and allowed in the past. Again, rules and restrictions may apply. Ask a copyrights lawyer.
3) Is it enforceable?
This is less about copyright law and more about international politics. Depends on what country and the owner of the copyright resides. For example, if you're in the US and the IP owner is in North Korea, what's the IP owner going to do? I'm Taiwanese. We use to have a lot, and I mean, A LOT of pirated software, CDs, DVD's, games, etc. But between international trade agreements and deals that happened in the last 2+ decades, a lot of that is clamped down and a fair amount is enforced. But if one country doesn't care and the other country can't get it to take action, what are they gonna do? I mean, look at all the IP theft happening in China.
All in all, a simple solution is to just ask the original owner if it's okay before doing it. Just having that okay makes navigating all this a lot easier.
